1. Field of the Invention
The present invention relates to a cholesteric liquid crystal color filter suitably used in displays such as LCDs and a process for producing the same. More specifically, the invention relates to a cholesteric liquid crystal color filter that has excellent color resolution in respective pixels, color reproducibility and display contrast as well as to a process for producing the same.
2. Description of the Related Art
Color filters used in, for example, color liquid crystal displays are generally composed of red (R), green (G) and blue (B) pixels, and a black matrix arranged therebetween for improving display contrast. Such color filters have generally been produced by spin-coating a glass plate with colored resin solutions in which respective pigments are dispersed in the resin for each color followed by patterning by photolithography, or by printing colored-pxels directly on a substrate.
Contact holes for connecting TFT elements to display pixels and openings for electrode lead-out portions are formed, separately after the colored pixels (and black matrix) have been formed, by forming a resist and subsequently dry-etching of the resist.
However, the printing method has a problem in that it is difficult to form highly accurate image patterns due to low resolution of the pixels, while the spin-coating method has a problem in that loss of materials is large and uneven coating occurs when a large area is coated. Further, since openings such as contact holes must be formed by forming a resist and subsequently dry-etching of the resist after colored pixels have been formed, the above methods are disadvantageous in that production costs are high. It is thus difficult to manufacture color filters at a low cost under the current circumstances.
Color filters made mainly of a liquid crystal material (particularly, a cholesteric liquid crystal) have widely been studied to enable efficient and easy manufacture of high-quality color filters having excellent transmittance and color purity with reduced loss of materials.
Since a color filter made mainly of a cholesteric liquid crystal (also referred to as a “cholesteric liquid crystal color filter” below) uses polarized light to effect image display by reflecting light of a predetermined wavelength and transmitting light of other wavelengths, the color filter can efficiently utilize light and has excellent transmittance and color purity. From the viewpoint of decreasing the number of steps and lowering costs in the production process with reduced loss of materials, a production process using a photoreactive chiral compound (chiral agent) is particularly useful. If a liquid crystal composition containing a photoreactive chiral agent is used, isomerization of the chiral agent progresses when the compound is patternwise irradiated with light of wavelengths to which the chiral agent is photosensitive, depending on the intensity of the irradiation energy (amount of light irradiated). This causes a change in helical pitch (twist angle of the helix) of the liquid crystal compound, whereby desired selectively reflected color light can simply be obtained by conducting patterning exposure having varied light quantities one time.
However, conventional cholesteric liquid crystal color filters have problems such as poor resolution, which is attributable to diffusion of a photoreactive chiral agent among pixels. Another problem is that, because the colored pixels and the black matrix (also referred to as “color filter layer” below) arc formed on the entire substrate surface, it is not easy to form contact holes and electrode lead-out portions per pixel. As an example of techniques relevant thereto, the assignee has previously proposed a method of forming contact holes (Japanese Patent Application No. 2001-070568), but this method cannot be conducted at a low cost.
To cope with the aforementioned problems, a method has been proposed in which, besides a liquid crystal compound, a polymerizable monomer is added to a liquid crystal composition and cured to form partition walls so that diffusion of the chiral agent can be prevented to thereby enhance resolution. However, adding a polymerizable monomer may sometimes significantly change the aligning property and phase transition temperature of the liquid crystal compound. Hence, it has been difficult to form color filters having sufficient reflective strength. To solve this problem, the assignee has previously proposed a method of forming partition walls by polymerizing a liquid crystal compound (Japanese Patent Application No. 2001-10534), but the formed partition walls may occasionally develop colors themselves by selectively reflecting visible light. If this proposed color filter is used as a reflective liquid crystal display element, contrast of the display element may be lowered. There has thus been a demand for additional improvement.
In a case where a film (layer) (e.g., a color filter layer) containing a cholesteric liquid crystal composition is formed by applying coating, an undesirable state occurs in that, even if treatment is conducted to align the liquid crystal molecules parallel to a substrate at a substrate side of the layer, the inclination angle (pre-tilt angle) of the liquid crystal molecules continuously changes in the thickness direction of the layer, particularly when using a low-molecular weight liquid crystal. Such a state is caused by the fact that one face of the color filter forming layer is exposed to the environment, whereby the liquid crystal molecules are oriented perpendicular to the substrate at the side exposed to air. Therefore, this layer usually needs be disposed between alignment layers. In a case where the cholesteric liquid crystal composition is polymerized for use as an optical film, it is necessary to peel off at least one of the above-described alignment layers after polymerization in order to achieve a light weight and thinness. Thus, there have been problems in that more steps are required, such as producing and removing the alignment layers, and larger amounts of waste are produced. As described above, there has not yet been provided a process for readily producing a color filter with improved transmittance and color purity, by which reduced loss of materials can be achieved, contact holes and openings can be formed at a low cost, and in which aligning can be carried out easily and efficiently.